FIELD OF THE INVENTION
The invention relates to a method for work-hardening a component to be loaded along a loading axis by rolling in a notch extending approximately at right angles to the loading axis, the notch being work-hardened by rolling along a plurality of tracks being adjacent to one another along the loading axis and being directed approximately at right angles to the loading axis, with each track covering only a part of the notch and compressive internal stresses being generated in the component below the notch.
The invention relates in particular to the work-hardening by rolling of metallic components such as turbine blades, where a notch in which such a component is to be work-hardened by rolling is, in particular, a retention groove for holding the component. A turbine blade, in particular, which is intended for a turbine rotor and is correspondingly subjected to large operational centrifugal loads, frequently has a plurality of retention grooves which are disposed along the loading axis in two mutually symmetrical rows and in which corresponding holding appliances of the turbine rotor engage.
Component work-hardening by rolling is summarized under the general term "surface precision rolling" together with finish-rolling which smooths the surface and dimensional rolling which slightly changes the shape. The term "surface precision rolling" is thoroughly explained in a book entitled "Festwalzen und Glattwalzen zur Festigkeitssteigerung von Bauteilen" [Hard-Rolling and Smooth-Rolling to Increase the Strength of Components] of the Deutschen Verbands fur Materialprufung e.V. [German Association for Material Testing]. Written versions or manuscripts of lectures which were given at the eighth "Arbeitskreises Betriebsfestigkeit" [Task Force on Operating Strength] meeting held in 1982 are collected in that book. Individual reference is made herein to the lectures "Ermudungseigenschaften festgewalzter Probestabe" [Fatigue Properties of Hard-Rolled Test Bars] by B. Fuchsbauer, Page 23 ff, and "Eigenspannungsausbildung und Dauerfestigkeit von festgewalzten CK45-Proben" [Intrinsic Stress Development and Long-Term Strength of Hard-Rolled Specimens of CK 45] by R. Prummer and R. Zeller, Page 63 ff. It can be learned from those sources that work-hardening by rolling, which is usually carried out with small, hard metal crowned rollers, is a treatment in which the surface is deformed with the objective of increasing the life, in particular the endurance under vibration crack corrosion attack. During work-hardening by rolling, the surface deformation is partly elastic and partly plastic. On one hand, it leads to an increase in the hardness of the treated material at the surface and in the layers close to the surface and on the other hand, it leads to the formation of compressive internal stresses under the surface. Under certain circumstances, those compressive internal stresses are suitable for preventing crack growth from the surface into the material. The suitability of compressive internal stresses for combatting vibration crack corrosion on the material follows therefrom.
The avoidance of danger due to vibration crack corrosion is an important aspect in the planning and manufacture of a turbine blade for a gas turbine or a steam turbine.
A method for work-hardening by rolling of the type mentioned in the introduction above is apparent from German Patent DE 40 15 205 C1, in which the component with a notch to be treated through the use of work-hardening by rolling is a turbine blade. The main feature in that case is not an embodiment of the work-hardening by rolling itself, but rather information on a tool that is suitable for the work-hardening by rolling of certain notches or grooves.
German Patent DE-PS 869 912 also relates to the process of work-hardening by rolling although it is not a component of the type mentioned in the introduction above which is work-hardened by rolling, but rather a flat circular ring representing a race of a rolling contact thrust bearing.
German Published, Non-Prosecuted Application DE 36 01 541 A1 relates to an improvement in the surface properties at a hole in a work piece through the use of work-hardening by rolling the surface at the hole with rolling balls. The work-hardening by rolling preferably takes place in such a way that the balls are guided along the axis of the hole and therefore roll over the hole on a multiplicity of tracks which are adjacent one another with each of them being directed approximately parallel to the axis. Compressive internal stresses, a substantial proportion of which are tangentially directed relative to the axis, can be generated in the work piece in that way.
The work-hardening by rolling of a notch in a component, which is known from the state of the art, always takes place in such a way that a crowned roller with a shape being matched to the shape of the notch is rolled through the notch with a corresponding contact force. It has been found that compressive internal stresses are then formed which are anisotropic and a major proportion of which are directed in the longitudinal direction of the notch. Those compressive internal stresses are therefore only suitable to a limited extent for combatting the operational loads on components of the type mentioned in the introduction above, in particular turbine blades, in which the loading takes place along a loading axis extending transverse to the notch.